1. Field of the Invention
The present invention relates to a method of producing a photovoltaic element, specifically a method of producing a photovoltaic element having excellent characteristics by removing short-circuit current paths due to defects.
2. Related Background Art
In recent years, with increased area of a photovoltaic element such as a solar cell, a multilayered photovoltaic element comprising a semiconductor film having a large area such as an amorphous silicon film have been developed. To produce photovoltaic elements with a large area, continuous process techniques such as a roll-to-roll technique are attracting attention.
However, it is difficult to produce a photovoltaic element with a multilayered structure which is free from defects over a large area. For example, in the case of photovoltaic elements comprising a plurality of stacked thin films of amorphous silicon or the like, pin-holes and defects caused by the effects of dust in the deposition of semiconductor layers are likely to cause shunts and shorts. These shunts and shorts are well known in significantly degrading the characteristics of photovoltaic elements. Particularly among these characteristics, it is known that photovoltage generation characteristics are significantly deteriorated.
The cause and effect of pin-holes and defects will be described in detail. For example, in the case of an amorphous silicon solar cell formed by depositing amorphous silicon on a stainless-steel substrate, the substrate surface can not be said to be a perfectly smooth and flat plane, but has flaws and hollows, or spike-shaped protrusions. In some cases, an electrode layer with projections and depressions (back reflector) is formed on the stainless-steel substrates to scatter light. Therefore, a large cause of generating defects and pinholes exists in that a thin semiconductor layer having a thickness of a few hundreds .ANG. such as an n-type or p-type layer can not completely coat the above surface. Also, as described above, minute dusts in a deposition process may cause pinholes.
In this way, a semiconductor layer between a first electrode (lower layer) and a second electrode (upper layer) in a solar cell is lost due to pinholes, so that the first electrode and the second electrode directly contact to each other, or spike-shaped defects on a substrate are in contact with the second electrode, or a semiconductor layer is not completely lost but a shunt or a short circuit having a low resistance is formed. In these cases, an electric current generated by light flows in parallel with the second electrode into the shunt portion or the short circuit portion having a low resistance, whereby the generated current is lost. When these current losses exist, the open circuit voltage of a solar cell, that is, the voltage characteristics are significantly reduced. This reduction is more remarkable under low light intensity, and thus becomes a serious problem for a solar cell which is required to generate electricity under any weather conditions.
In the case of the short circuit described above, it is required to take counter-measures for minimizing the current loss caused by the current flowing into the low resistance portion forming the short circuit. For such counter-measures, it is well known to minimize or eliminate the current loss either by directly removing defect portions and pinholes or by removing materials on the periphery of short circuit portions or by changing the materials into electrical insulating materials.
Specifically, U.S. Pat. No. 4,166,918 discloses a method of removing a defect portion in an electrical short circuit portion in a solar cell having a large area, in which the defect portion in the solar cell is baked and removed by applying a sufficiently high reverse bias voltage not more than breakdown voltage. However, the application of a high reverse biased voltage to a solar cell includes the possibility of damage to the normal portion other than the defect portion during baking, and the controlling is difficult.
Further, Japanese Patent Publication No. 62-59901 discloses a method of making up a pinhole in a semiconductor device by using a laser beam. However, this technique requires special application for focusing a laser beam on the pinhole. Further, there is possibility of damage to a normal portion other than pinholes by the laser beam.
Furthermore, Japanese Patent Publication No. 62-4869 discloses a method of filling a pinhole extending through an amorphous film of a photovoltaic element with electrical insulating materials. After the applying a photosensitive insulating material to the film, the pinhole is filled with the insulating material by selectively exposing the pinhole portion to light through an optically transparent substrate. Therefore, this method can not be applied to the cases of an opaque electroconductive substrate and so the range of application is limited.
Still further, U.S. Pat. No. 4,729,970 discloses a method of electrically insulating a short defect portion in an electric device including a transparent electroconductive film from the electrode by bring a conversion reagent into contact with the short defect portion to change the electroconductive film in the vicinity of the defect portion to a high resistant material. Specifically, the method comprises a treatment using a reagent containing a Lewis acid (more definitely, a salt of amphoteric elements), to be precise, chlorides such as AlCl.sub.3 and ZnCl.sub.2. However, in the cases of the treatment using such a reagent of amphoteric elements, when a photovoltaic element having a multilayered film structure includes an amphoteric metal such as Al, the amphoteric metal is significantly eroded to result in a problem of such a side effect of peeling off during the treatment.
Yet further, U.S. Pat. Nos. 5,084,400 and 5,320,723 discloses a method of electrically insulating a short defect portion in the photovoltaic element deposited on a metal substrate from the electrode, by applying a voltage to the electroconductive film in the vicinity of the short defect portion in an acid solution such as H.sub.2 SO.sub.4 to change the electroconductive film to a high resistance film. This method has certainly overcome the problem of peeling off with respect to Al as described above by using an acid having sulfate group such as H.sub.2 SO.sub.4. However, in the cases of photovoltaic element composed of a thin film of such a material as ZnO, as the erosion of ZnO is very sensitive to the concentration of hydrogen ions, the problem of peeling off during the treatment exists.